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Research Papers: Multiphase Flows

Water Droplet Adhesion on Hydrophobic Surfaces: Influence of Droplet Size and Inclination Angle of Surface on Adhesion Force

[+] Author and Article Information
Abdullah Al-Sharafi

Department of Mechanical Engineering,
King Fahd University of Petroleum and Minerals,
Dhahran 31261, Saudi Arabia
e-mail: alsharafi@kfupm.edu.sa

Bekir S. Yilbas

Department of Mechanical Engineering;Centre of Excellence for Renewable Energy,
King Fahd University of Petroleum and
Minerals,
Dhahran 31261, Saudi Arabia
e-mail: bsyilbas@kfupm.edu.sa

Haider Ali

Department of Mechanical Engineering,
King Fahd University of Petroleum and Minerals,
Dhahran 31261, Saudi Arabia
e-mail: haiali@kfupm.edu.sa

1Corresponding author.

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received October 12, 2016; final manuscript received February 15, 2017; published online June 5, 2017. Assoc. Editor: Shizhi Qian.

J. Fluids Eng 139(8), 081302 (Jun 05, 2017) (13 pages) Paper No: FE-16-1672; doi: 10.1115/1.4036166 History: Received October 12, 2016; Revised February 15, 2017

Adhesion of various size sessile droplets on the hydrophobic surfaces is considered, and the moment generated about the locus of the droplet meniscus is determined for several inclination angles of hydrophobic surface. An experiment is designed to examine the influence of inclination of hydrophobic surface on the water droplet behavior. The flow field generated inside the droplet is simulated to predict the flow acceleration and its effects on adhesion force. Simulations are repeated for different inclination angles of hydrophobic surface. The flow predictions are validated through the experimental data. It is found that the moment about the locus of droplet meniscus increases with increasing inclination angle, which is more pronounced for the large volume water droplets, such as ∀ = 45 μL; however, further increase of inclination angle lowers the moment because of significant change of the location of the line of action of the total force during the excessive body deformation of the droplet. The flow field developed inside the droplet forms a circulation cell, and the orientation and size of the circulation cell change with droplet volume, which becomes significant at high inclination angles. The flow acceleration inside the droplet does not have significant contribution to the overall force generated on the droplet during the inclination of the hydrophobic surface. The shear force generated at the wetted surface of the droplet plays in significant role on the adhesion force.

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Figures

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Fig. 1

Images of droplets prior and after surface inclination. ∀ is the droplet volume, and δ is the inclination angle of the plate.

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Fig. 2

Velocity vectors in the flow field inside droplet for droplet contact angle of 100 deg: (a) three-dimensional simulation and (b) two-dimensional simulation

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Fig. 3

SEM micrographs of solvent crystallized PC surface: (a) crystallized surface area, (b) poles formed after crystallization, and (c) three-dimensional optical image of the crystallized surface

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Fig. 4

Experimental setup for particle analysis in the droplet

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Fig. 5

Particles recorded and numerically simulated inside droplet: (a) particles captured on the microscopic camera for two time frames and (b) simulation particles trajectory

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Fig. 6

AFM image of the crystallized PC surface: (a) three-dimensional image of the surface texture and (b) line scan of the textured surface

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Fig. 7

Schematic view of forces acting on the droplet

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Fig. 8

Normalized adhesion force, (Fad*=Fad/Wd) where Fad is determined from Eq. (8), with inclination angle for various droplet volumes

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Fig. 9

Moment of total forces acting on droplet about point A

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Fig. 10

Droplet images for various inclination angles and droplet volumes

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Fig. 11

Droplet deformation parameters with inclination angle for various droplet volumes

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Fig. 12

Velocity vectors inside the droplet of 5 μL volume for different bending angles

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Fig. 13

Velocity vectors inside the droplet of 45 μL volume for different bending angles

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Fig. 14

Normalized velocity (V*=V/μ2g/ρσ) of the droplet surface with different inclination angles for various droplet volumes

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